The present invention relates to a method of casting and a casting machine, more precisely relates to a method of casting and a casting machine, in each of which a molten metal is poured into a cavity of a casting die so as to cast a product having a prescribed shape.
Many kinds of ways of aluminium casting. For example, gravity casting has some advantages: simple casting dies, high quality products, etc.
The casting die of the gravity casting is shown in FIG. 10. In FIG. 10, the casting die 100 is made of a metal. The casting die 100 is a splittable die constituted by a lower die 102a and an upper die 102b. A cavity 104, in which a product having a prescribed shape is cast, is formed by the lower die 102a and the upper die 102b. 
The upper die 102b includes: a metal inlet 106, from which a molten metal, e.g., a molten aluminium, a molten aluminium alloy, is poured into the cavity 104; a feeder head 108 provided between the metal inlet 106 and the cavity 104; and air ventilation holes 110, from which air in the cavity 104 is discharged when the molten metal is poured into the cavity 104.
When the molten aluminium or aluminum alloy is solidified, its volume is reduced about 3% due to shrinkage. The shrinkage of the solidified metal in the cavity occurs a surface sink, etc. in the cast product. In the casting die 100 shown in FIG. 10, a gap, which is formed in the cavity 104 by the shrinkage of the solidified metal, is filled with a part of the molten metal in the feeder head 108, so that the defect, e.g., the surface sink, can be prevented.
Surface tension of the molten aluminium or aluminium alloy is made greater by an oxide film formed on a surface of the molten aluminium or aluminium alloy. Therefore, fluidity and running property of the molten aluminium or aluminium alloy are low, and smoothness of a surface of the product is also low. To solve these disadvantages, inner faces of the feeder head 108 and the cavity 104 of the casting die 100 shown in FIG. 10 are coated with lubricant so as to improve the fluidity and the running property of the molten metal whose surface is covered with the oxide film.
When the product is cast by the casting die 100 shown in FIG. 10, the molten aluminum or aluminium alloy is poured into the metal inlet 106 of the casting die 100. The cavity 104 and the feeder head 108 are filled with the molten aluminium or alulminium alloy with discharging the air from the air ventilation holes 110.
Next, the casting die 100, in which the molten metal has been filled, is cooled so as to solidify the molten metal in the cavity 104. By the solidification of the molten metal in the cavity 104, the solidified metal is shrinked and the gap is formed in the cavity 104, but the gap in the cavity 104 is filled with the molten metal supplied from the feeder head 108.
However, in the conventional method of aluminium casting shown in FIG. 10, the inner faces of the feeder head 108 and the cavity 104 must be coated with the lubricant so as to improve the fluidity and the running property of the molten metal whose surface is covered with the oxide film. But, it is very difficult for inexperienced workers to define coating portions and to uniformly form coating layers. Therefore, surface defects of cast products, e.g., rough surfaces, cannot be avoided.
The inventors of the present invention invented and filed an improved method of aluminum casting (Japanese Patent Application No. 2000-108078), in which aluminium products having good and smooth surfaces can be cast without coating any lubricant.
The improved method will be explained with reference to FIG. 11. Firstly, a magnesium nitride compound (Mg3 N2), which is an example of deoxidizing compounds, is introduced into the cavity 104 of the casting die 100, then the molten aluminium or aluminium alloy is poured therein.
In the improved method, the deoxidizing compound is previously existed in the cavity 104 of the casting die 100, so that the oxide film formed on the surface of the molten aluminium or aluminium alloy can be deoxidized and the surface tension of the molten aluminium or aluminium alloy can be made lower. By deoxidizing or removing the oxide film, the fluidity and the running property of the molten metal can be improved, so that surfaces of the cast products can be smooth and can have good external surfaces.
The feeder head 108 shown in FIG. 10 or 11 is capable of filling the gap, which is formed in the cavity 104 when the solidified metal is shrinked, with the molten metal. Therefore, at least a part of the molten metal in the feeder head 108 must have enough fluidity, even if the molten metal in the cavity 104 is solidified.
Namely, solidifying speed of the molten metal in the feeder head 108 must be lower than that of the molten metal in the cavity 104. Thus, cooling rate of the feeder head must be lower than that of the cavity. To make the difference of the cooling rate, the feeder head 108 is formed into, for example, a pillar shape having broad traverse sectional area. By the pillar-shaped feeder head 108, the molten metal in the feeder head 108 is not easily cooled.
However, the solidified metal in the feeder head 108 is a disused part, so it will be removed from the product. If the solidified metal in the feeder head 108 is reused, it must be molten and energy must be consumed.
Therefore, the pillar-shaped feeder head 108, which has broad traverse sectional area, has greater volume, so yield of casting material must be lower and energy consumption for reuse must be greater.